1. Field
Circuit fabrication, including a method for forming a base region of a bipolar transistor.
2. Description of the Related Art
Bipolar transistors, more specifically heterojunction bipolar transistors (HBTs) are used in devices requiring high frequency operation such as wireless and networking devices. HBTs are used in these devices because of their high cut off frequencies greater than 150 gigahertz (Ghz) even though they consume more power than equivalent metal oxide semiconductor (MOS) based technologies.
HBTs typically consist of an emitter region, base region and collector region. The emitter region generally has a larger band gap than the base region to achieve high frequency performance. The speed at which the HBT can switch is referred to as the cutoff frequency, ft. The cutoff frequency of a given HBT is generally related to the width of its base region. The narrower the base region of a HBT, the shorter the base transit time and higher the cutoff frequency, ft.
HBTs formed in silicon germanium (SiGe) films typically use boron diffusion or implantation to form a base region. Methods for forming HBTs include chemical vapor deposition (CVD) techniques. However, current methods for forming HBTs are limited in further reliably decreasing the base width because such methods are generally unable to create adequate boron concentration in a base region with reduced base width while controlling the SiGe deposition rate. Boron tends to diffuse rapidly in a SiGe film using current methods causing thick base widths of greater than 25 nanometers (nm).